But Cornyn isn't convinced O’Rourke has given up his Senate aspirations.

On Tuesday, he sent donors an email blast warning of “Beto’s Texas,” hinting that the El Paso Democrat could yet come after him, and asking for help filling a new "Stop Beto Fund."

“I don't think it's out of the realm of the possibility that that could happen,” Cornyn said Wednesday when asked about his fundraising message. “The filing deadline is December the 9th, I believe. So my expectation is that perhaps Beto, perhaps Julian Castro or others who have indicated that they're running for president — if they're not getting a lot of traction then obviously it's very easy to pivot into the Senate race.”

In November, O’Rourke, who served six years in the U.S. House, stunned Texas Republicans by holding Sen. Ted Cruz below 51 percent in a nail-biter contest in which he raised a stunning $80 million, the most of any Senate candidate ever. Democrats haven’t won a statewide race in Texas since 1994, and that was the closest any had come.

Presidential buzz began immediately. He reportedly has been gearing up to launch a White House bid. Last week, he promised to announce his plans “soon.”

Other Texas Democrats are considering a run against Cornyn, including Rep. Joaquin Castro of San Antonio; Wendy Davis, a former state senator who lost to Gov. Greg Abbott by over 20 percentage points in 2014; and MJ Hegar, an Iraq war veteran who narrowly lost a bid for Congress last fall.

“I'm not very nervous,” Cornyn said when asked about those three potential challengers.

Davis told the Associated Press that she won’t run if Castro does, but is otherwise thinking about it.

“I said that I would consider it and I'm considering it,” Castro told The Dallas Morning News on Wednesday. As for Davis, he said: "She is a great friend and we had a good conversation the other day. As she mentioned, we'll have a strong candidate for Texas in 2020."

Castro’s profile has exploded in recent months as he became the chairman of the Congressional Hispanic Caucus. He was point man in the House on the resolution of disapproval over President Donald Trump's effort to build a border wall by invoking emergency presidential authority.

He's also chairing his twin brother’s presidential campaign. Julián Castro is a former San Antonio mayor and was housing secretary in the second term of Barack Obama.

There is some irony in the chain of events: O’Rourke’s focus on the White House would mean there are two Texas Democrats in the hunt. That complicates matters for Julian Castro, even as he clears a path toward the Cornyn seat for his brother.

Senate Minority Leader Chuck Schumer, the top Democrat in the Senate, had urged O’Rourke to run against Cornyn.

After O’Rourke decided against it, Schumer met with Hegar, who lost to Rep. John Carter, R-Round Rock, by about 8,000 votes out of 281,000.

Nearly 3 million people have viewed a 3-minute campaign video that Hegar, a decorated Air Force helicopter pilot, used in her effort to unseat Carter.